What Is a POST Test Card?

A POST test card displays error codes generated during the Power On Self Test. It's used to identify problems that can be detected as the computer is starting up, so it's helpful when your computer won't turn on.

POST codes correspond directly to a test that has failed and can help determine what piece of hardware causes problems, regardless if it's the memory, hard drives, keyboard, etc.

If the system doesn't encounter an error until later on during the boot process after the video card is activated, then the error can be displayed on the screen. This type of error isn't the same as a POST code, but instead is called a POST error message, which is a human-readable message.

How POST Test Cards Work

Most POST cards (aka, checkpoint cards or port 80h cards) plug directly into expansion slots in the motherboard, while a few others connect externally through a parallel or serial port. An internal POST test card, of course, requires you to open your computer to use it.

During the POST, a two-digit code transmits on port 0x80. Some cards include jumpers that let you modify which port to read the code from, since some manufacturers use a different port.

This code is created during each diagnostic step during bootup. After each piece of hardware is identified as working, the next component is checked. If an error is detected, the boot process usually halts, and the POST test card shows the error code.

For example, if the POST test card shows the error number 28, and Dell is the BIOS manufacturer, it means that the CMOS RAM battery has gone bad. In this case, replacing the battery would most likely fix the problem.

More About POST Test Cards

Since the BIOS can deliver an error message before the video card is enabled, it's possible to experience a hardware problem before the monitor can display the message. This is when a POST card comes in handy—if the error can't be delivered to the screen, the card can still help identify the problem.

Another reason to use one is if the computer is incapable of making a sound to give the error, which is what beep codes are. They are audible codes that correspond to a particular error message. While they're useful when an error message can't be displayed on the screen, they're not at all helpful on computers that don't have an internal speaker, in which case the corresponding POST code can be read from a POST test card.

Few people already own one of these testers, but they're not very expensive. Amazon sells many types of POST cards, many of which cost less than $20 USD.